1. Field of the Invention
This invention relates to a solid oxide fuel cell.
2. Related Art Statement
Recently, fuel cells have been recognized as power generating equipment. The fuel cell is a deuce capable of directly converting chemical energy possessed by fuel to electrical energy. Since the fuel cell is free from the limitation of Carnot's cycle, it is an extremely promising technique in that the fuel cell essentially has a high energy conversion efficiency, various fuels (naphtha, natural gas, methanol, coal reformed gas, heavy oil, etc.) may be used, the public nuisance is less, the power generating efficiency is not influenced by the scale of the equipment.
Particularly, since the solid oxide fuel cell (hereinafter abbreviated as SOFC) operates at a high temperature of 1000.degree. C. or more, the activity of the electrode is very high, and the use of a noble metal catalyst such as expensive platinum is not required. In addition, since the SOFC has a low polarization and a relatively high output voltage, the energy conversion efficiency is considerably higher than that of other fuel cells. Furthermore, since the SOFC is constructed with solid materials, it is stable and has a long use life.
A cell unit for SOFC is generally comprised of an air electrode, a solid electrolyte and a fuel electrode.
A flat plate type SOFC cell unit is large in the effective cell area per unit volume and thus shows promise. It is known that a plurality of such flat plate type SOFC cell units are arranged in parallel with each other and rigidly and closely fixed to each other to form a power generation chamber, whereby an oxidizing gas and a fuel gas are supplied from one side of the power generation chamber and a burnt exhaust gas is discharged from the other side thereof.
However, when the cell units are rigidly and closely fixed to each other to form an airtight power generation chamber, these units are at a mutually sealed and restrained state, so that a large thermal stress occurs in an edge portion of the cell unit at a high temperature in the operation. Furthermore, electrode reaction is active in the vicinity of a supply port for the oxidizing gas and fuel gas, while electrode reaction is inactive in the vicinity of a discharge port for the exhaust gas and the temperature at this port is low, so that a large temperature gradient is caused in the power generation chamber, whereby a large thermal stress is also created. These thermal stresses are apt to produce cracks in the brittle fuel cell unit, and consequently the power generation efficiency lowers and the breakage of an assembly of these cell units proceeds.
Particularly, when the base portions of the SOFC cell units are rigidly supported by a partition wall arranged between the oxidizing gas supply chamber and the exhaust gas combustion chamber, the pushing stress becomes larger and hence cracks are also apt to be produced in the brittle cell unit.